Barrel plating in which objects are tumbled in a perforated horizontal rotating drum is a common method of electroplating small parts. Representative technology is disclosed in U.S. Pat. No. 4,822,468 by Kanehiro and U.S. Pat. No. 4,769,117 by Shino, et al. Many very small parts cannot be plated effectively in a barrel due to poor contact with the current feeder or fouling on the interior of the drum. These problems often necessitate the addition of plating media (typically some type of smooth metal shot) to the barrel to improve cathodic contacting and part motion.
The use of media significantly increases the required plating time and current because the media is also plated and therefore, the plating cost per part is increased. Additionally, many small parts are fragile or can interlock and may be damaged by tumbling with heavy media. Consequently, these parts cannot be plated successfully in barrels.
U.S. Pat. No. 5,487,824 by Greigo discloses an integrated electroplating system designed specifically to electroplate very small parts which employs a horizontal accelerating rotating drum to maintain a packed bed of parts in motion during electroplating.
U.S. Pat. No. 3,654,098 by Backhurst et al. and U.S. Pat. No. 3,703,446 by Haycock et al. disclose fluidized bed cathodes. Although fluidized beds have excellent liquid-solid contacting, fluidized bed cathodes suffer from poor electrical contact between the fluidized particles, non-homogeneous electrical potentials and particle segregation effects. Additionally, it is difficult to maintain the entire bed fluidized when the particles are changing in size, and possibly density, due to metal deposition. It is unlikely that the potential benefits of the fluidized bed approach will be realized in a practical electrodeposition system.
Typical spouted beds consist of a cylindrical vessel with a conical bottom section. The vessel contains a bed of particles which form the spouted bed. Fluid is introduced into the spouted bed vessel at the bottom of the conical section as a jet. This fluid jet penetrates the bed of particles contained in the spouted bed vessel, entraining particles and forming a "spout" of upward moving particles and fluid. The particles disengage from the fluid flow in a region above the particle bed and then fall on top of the downward-moving annular bed. The "pumping action" provided by the spout circulates the particles through the vessel in a torroidal fashion; upwards in the spout and downwards in the annular moving bed. A "draft pipe" may be incorporated into the vessel to assist in the fluid transport of the particles. The draft pipe consists of a tube which is fixed coincident with the location of the spout, directly above and aligned with the liquid jet. The draft pipe delays the dispersion of the liquid jet and allows particle transport over a broader range of fluid velocities while also stabilizing the liquid flow.
U.S. Pat. No. 4,272,333 by Scott discloses the use of a moving bed electrode (MBE), in which conductive particles move downward vertically in a packed bed between two electrodes, the anode being shielded with a membrane. The necessity of using a membrane to shield the anode makes this configuration less attractive for practical applications, since the mechanical abrasion of the moving bed of particles can damage the membrane in a short time. Additionally, metal deposition on the membrane may be a complication.
An article by Hadzismajlovic et al. published in Hydrometallurgy, Vol. 22, pages 393-401 (1989), and U.S. Pat. No. 1,789,443 by Levin disclose the use of spouted bed cathodes with anodes suspended above the spouted bed surface. Although this configuration may eliminate the complication of shielding electrodes using membranes, several operational problems may be encountered with this configuration. Many electrolytes have poor electrical conductivity; therefore, it is advantageous to have the cathode and anode in close proximity in order to reduce the voltage drop over the cell. This cannot be accomplished in these prior art systems, since the spout would collide with the anode. Additionally, the projected spouted bed geometric surface area is very limited, impairing electrode performance.
Conventional spouted beds also suffer from a particle recirculation problem commonly referred to as "dead spots", where a portion of the particle bed is stagnant. Dead spots usually exist at the outer edge of the spouted bed surface and are attributable to a failure of the spout to deposit particles at the circumference of the spouted bed. In an attempt to remedy this problem, spouted beds with very steep bottom cone angles have been adopted. In all cases, the radius of the spouted bed has been strictly limited to the distance to which particles in the spout can be transported radially outward by the fluid flow.